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HSI, HSE, LSI, LSE and clock nonsense

Question asked by Rami Rosenbaum on Feb 22, 2017
Latest reply on Feb 24, 2017 by Clive One


I'm working on a stm32F101CBT6.

The board has a 16 MHz HSE (connected to PD0.PD1) and a 32.768 KHz LSE (connected to PC14/PC15).

The board will use SPI1, SPI2, USART1, USART2, I2C1 and a few GPIOs.


I've worked with ST microcontrollers before, but never had to configure the clocks, so I'll be probably asking some naive questions, please be gentle... I do know that external oscillators are much more precise than the internal ones.


When creating a new project with CubeMX, it defaults to HSI and LSI.

If I enable MCO (PA8) and listen with an analyzer - I get 8 MHz. That seems correct.


1. If I configure the clock configuration, via CubeMX, to use HSE with 16 MHz (the board schema says 16 MHz) - I see on MCO 16.67 MHz. Is that correct?


2. What is the logic, the aim of playing with the PLL, and - should I use it?

My common sense says I should aim for the highest frequency in the SYSCLK and the buses.


I've attached my CubeMX project file, if it helps.